Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene and carbon nano-tubes. The carbon nano-tube refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphite sheet (a sheet of graphene plane or basal plane) or several graphite sheets to form a concentric hollow structure. A graphene plane is characterized by having a network of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers.
Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the helical tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical, thermal and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, coating ingredients, solid lubricant, fillers for a resin, and composite reinforcements.
Iijima was the first to report the production of carbon nanotubes by an arc discharge between two graphite rods. This technique still remains to be the most commonly used technique for producing carbon nanotubes; however, yield of pure carbon nanotubes with respect to the end product is only about 15%. Thus, a complicated, slow and expensive purification process must be carried out for particular device applications.
Kusunoki described another conventional approach to produce carbon nanotubes, which was published in an article entitled “Epitaxial Carbon Nanotube Film Self-organized by Sublimation Decomposition of Silicon Carbide” (Appl. Phys. Lett. Vol. 71, pp. 2620, 1977). Carbon nanotubes were produced at high temperatures by irradiating a laser onto graphite or silicon carbide. In this case, the carbon nanotubes are produced from graphite at about 1,200° C. or more and from silicon carbide at about 1,600 to 1,700° C. However, this method also requires multiple stages of purification which increases the cost. In addition, this method has difficulties in large-device applications.
Li, et al. Reported a method of producing carbon nanotubes through a thermal decomposition of hydrocarbon series gases by chemical vapor deposition (CVD) (“Large-Scale Synthesis of Aligned Carbon Nanotubes,” Science, Vol. 274, Dec. 6, 1996, pp. 1701–1703). This technique is applicable only with a gas that is unstable, such as acetylene or benzene. For example, a methane (CH4) gas cannot be used to produce carbon nanotubes by this technique.
A carbon nanotube layer may be grown on a substrate using a plasma chemical vapor deposition method at a high density of 1011 cm−3 or more. The substrate may be an amorphous silicon or polysilicon substrate on which a catalytic metal layer is formed. In the growth of the carbon nanotube layer, a hydrocarbon series gas may be used as a plasma source gas, the temperature of the substrate may be in the range of 600 to 900° C., and the pressure may be in the range of 10 to 1000 mTorr.
In summary, carbon nano-tubes are extremely expensive due to the low yield and low production and purification rates commonly associated with all of the current carbon nano-tube preparation processes. The high material costs have significantly hindered the widespread application of nano-tubes. A large number of researchers are making attempts to develop much lower-cost processes for nano-tubes. We have taken a different approach in that, instead of carbon nano-tubes, we chose to develop alternative nano-scaled carbon materials that exhibit comparable properties, but are more readily available and at much lower costs.
It is envisioned that individual nano-scaled graphite planes (individual sheets of graphene plane) and clusters of multiple nano-scaled graphene sheets, collectively called “nano-sized graphene plates (NGPs),” could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate (FIG. 1). Studies on the structure-property relationship in isolated NGPs could provide insight into the properties of a fullerene structure or carbon nano-tube. Furthermore, these nano materials could potentially become cost-effective substitutes for carbon nano-tubes or other types of nano-rods for various scientific and engineering applications.
Direct synthesis of the NGP material had not been possible, although the material had been conceptually conceived and theoretically predicted to be capable of exhibiting many novel and useful properties. The present invention provides a process for producing large quantities of NGPs. The process is estimated to be highly cost-effective.